Cast-iron cookware can react with liquid and/or acidic foods like tomato sauce and leach small amounts of iron into the food. But this is not necessarily a bad thing. Cooking in cast iron can be an easy way to increase your iron intake, especially if you don't eat meat. (See this blog post for more on Cooking with Cast Iron).
One more tip: Don't cook recipes calling for artichokes in a cast-iron pan. The phenolic compounds in artichokes react with the iron to turn these vegetables into a harmless but unappetizing shade of gray!